In this study, the performances of end mills used widely in machining were investigated in case of their reuse by sharpening. For this purpose, at the CNC vertical machining center, slot milling processes were carried out with HSS end mills (used once) on the AISI 1050 manufacturing steel having two different diameters and with carbide end mills (again used once) on the AISI D2 (2080) cold work tool steel having two different diameters. Machining experiments were made by using four different cutting speeds, four different feed and four different cutting depths. These end mills, afterwards, were re-sharpened by considering their original tool geometry and the machining experiments were repeated at the same conditions and their performances were evaluated in comparison with the performances of the new tool. In general, wear amounts in the sharpened tools were observed to be higher with respect to the new tools. In the HSS tools, no adverse effect of sharpening process was observed on the machining performance. In the carbide tools, especially from the point of surface quality for the tools having small diameter there is no adverse effect of re-sharpening and re-sharpening can be recommended with the condition of observing the surface quality in parallel to the increase in diameter.